The evolution of shell form in tropical terrestrial microsnails

Mollusca form an important animal phylum that first appeared in the Cambrian, and today is,after Arthropoda, the second largest animal phylum, with more than 100,000 extant species(Bieler, 1992, Brusca and Brusca, 2003), with the class Gastropoda accounting for 80% of the extant species in the Mollusca. Despite its species-richness, a generalised gastropod shell architecture is maintained because of conserved developmental processes. All of the shelled gastropods grow by adding, in a unidirectional accretionary way, shell material with the mantle edge organ, usually at different deposition rates around the existing aperture. This shell ontogeny, or to be more specific aperture ontogeny, gives the general spiral form for the shells. However, spiral forms can vary when there are changes in any one of the aspects in the aperture ontogeny profiles, namely, the rate and direction of shell deposition around the aperture, size and shape of the aperture (i.e. mantle edge), and the total length of the shell ontogeny processes. The interplays between these developmental parameters have generated a great diversity in shell form, for which taxonomists and evolutionary biologist are now trying to accurately characterise and to understand with regard to its evolution.This thesis reveals several hitherto unknown aspects of Plectostoma shell forms,in terms of the developmental homology, the aperture ontogeny profile, anti-predation functionality, and evolutionary pattern in shell characters and ontogenetic morphospace evolution. In fact, these are the issues that have been targeted by biologists for centuries in order to improve the way shell shape is characterised and to improve understanding of shell form evolution.